A heat-absorbing glass plate is required to have a low solar transmittance and a high visible light transmittance. That is, it is required to have a high ratio Tv/Te of the visible light transmittance (by illuminant A, 2° visual field) as defined in JIS R3106 (1998) (hereinafter sometimes referred to as Tv, calculated as 4 mm thickness in the present invention) to the solar transmittance as defined in JIS R3106 (1998) (hereinafter sometimes referred to as Te, calculated as 4 mm thickness in the present invention).
In general, when the amount of total iron as calculated as Fe2O3 as represented by mass % based on oxides (hereinafter sometimes referred to as t-Fe2O3 so as to be distinguished from the amount of trivalent iron) is large, the solar transmittance tends to be low and the value of Tv/Te thereby tends to be high, however, the visible light transmittance is low at the same time. If the amount of total iron is small, although the visible light transmittance tends to be high, the solar transmittance tends to be high and the value of Tv/Te tends to be low.
As a heat-absorbing glass plate, for example, Patent Document 1 has been proposed. That is, a heat-absorbing glass plate which is made of soda lime silica glass having a Redox of from 0.38 to 0.60, containing from 0.005 to 0.18% of SO3 as represented by mass % based on oxides, and containing substantially no polysulfide, and which contains, as coloring components, any one of I) to V) as represented by mass % or by mass ppm:
I) total iron as calculated as Fe2O3: 0.6 to 4%,
FeO: 0.23 to 2.4%,
CoO: 40 to 500 ppm,
Se: 5 to 70 ppm,
Cr2O3: 15 to 800 ppm,
TiO2: 0.02 to 1%.
II) total iron as calculated as Fe2O3: 0.4 to 1%,
CoO: 4 to 40 ppm,
Cr2O3: 0 to 100 ppm.
III) total iron as calculated as Fe2O3: 0.9 to 2%,
FeO: 0.34 to 1.2%,
CoO: 90 to 250 ppm,
Se: 0 to 12 ppm,
TiO2: 0 to 0.9%.
IV) total iron as calculated as Fe2O3: 0.7 to 2.2%,
FeO: 0.266 to 1.32%,
Se: 3 to 100 ppm,
CoO: 0 to 100 ppm.
V) total iron as calculated as Fe2O3: 0.9 to 2%,
FeO: 0.34 to 1.2%,
CoO: 40 to 150 ppm,
Cr2O3: 250 to 800 ppm,
TiO2: 0.1 to 1%.
Further, Patent Document 2 discloses a heat-absorbing glass plate which has a solar transmittance of at most 42% calculated as 4 mm thickness, which has a visible light transmittance (by illuminant A, 2° visual field) of at least 70% calculated as 4 mm thickness, which provides a transmitted light having a dominant wavelength of from 492 to 520 nm, and which is made of soda lime silica glass having substantially the following composition, as represented by mass % based on oxides:
SiO2: 65 to 75%, Al2O3: more than 3% and at most 6%, MgO: at least 0% and less than 2%, CaO: 7 to 10%, total iron as calculated as Fe2O3: 0.45 to 0.65%, and TiO2: 0.2 to 0.8%, and containing substantially no member selected from the group consisting of CoO, Cr2O3, V2O5 and MnO, wherein the ratio (Redox) of the mass of divalent FeO to the mass of total iron which is the amount of total iron as calculated as Fe2O3 is higher than 42% and at most 60%.
Further, Patent Document 3 proposes a colored glass plate made of alkali-containing silica glass containing iron, tin and sulfur, wherein the amount of total sulfur as calculated as SO3 as represented by mass % based on oxides is at least 0.025%, the ratio (Redox) of divalent iron as calculated as Fe2O3 to total iron as calculated as Fe2O3 is from 60 to 80% as represented by mass %, and the proportion of divalent tin in total tin is at least 0.1% as represented by mol %.